For decades, television programs have optionally included closed caption (CC) information. Closed captions are rows of text that can be optionally displayed on a television picture. The two main uses of closed captions are to present a visual representation of the audio track for hearing-impaired viewers and to present a visual representation of the audio track in a different language.
Since the 1970s the United States Federal Communications Commission (FCC) has required closed captions to be included with television broadcasts in many instances. Closed caption data includes text to be displayed and other information, such as control codes, that specify the location and/or appearance of the text. Today, this closed caption data can be inserted into a television broadcast in a number of ways. Previously, there was only one way to insert closed caption data into a television broadcast. Namely, CC data was transmitted using a special line of National Television Standards Committee (NTSC) format video (line 21) that is transmitted to televisions but that is not displayed. At the television transmitter, the closed caption data is converted to intensity pulses in this special video line. The receiving television recognizes the pulses in this special line of video and converts the pulses back into closed caption data. The television then interprets the closed caption data and displays the captions (text) on the picture.
CC data is carried in two fields, CC Field 1 and CC Field 2. CC Field 1 carries two interleaved channels, CC1 and CC2, while CC Field 2 carries two additional interleaved channels, CC3 and CC4. Multiple CC channels are defined, so that a program can be encoded, for example, with CC data for multiple languages. To separate the interleaved data into individual data streams, such as CC1 and CC2 data streams, the data bytes are interpreted into control codes and characters as specified in the Consumer Electronics Association (CEA) standard CEA-608-C.
In recent years, there have been a number of technological changes in how television programs are stored and transmitted. The most fundamental change is that digital storage and transmission of television signals has largely replaced analog storage and transmission. Analog storage (VHS tapes, for example) and analog transmission (over-the-air antenna, for example) have been replaced almost completely by digital storage (DVD for example) and digital transmission (“Digital Cable” and satellite for example). The digital formats generally do not include the special line of video that analog broadcasts use to transmit closed caption data, so a new method of carrying closed caption data was needed. The MPEG2 compression format, which until recently was used by essentially all digital broadcasts for transmitting video, allows for optionally including digital closed caption data as user data with every frame. Other digital formats include serial digital interface (SDI), which is an uncompressed digital signal transmitted at 270 Mb/sec, and which is typically used for connecting video equipment within a video processing facility, for example a television studio. Asynchronous serial interface (ASI) is a compressed digital signal that operates at 270 Mb/sec and contains one or more compressed video streams.
In the mid 1990s, a new closed caption format referred to as DTVCC (Digital Television Closed Captions) was created, and is defined by the standard CEA-708-B. DTVCC takes advantage of the additional capacity available in digital transmissions, which allows roughly ten times as much caption data to be transmitted compared to CC data. As a result DTVCC can be used to display similar and more elaborate captions than CC. DTVCC data is organized into 63 channels or “services,” which are interleaved together. To separate the data into constituent channels (or services) the DTVCC data is interpreted according to the standard CEA-708-B. As the FCC continues to require broadcasters to transition to digital television (DTV) broadcasts, more programs are being created that include DTVCC captions.
However, many consumers still do not have a television capable of displaying DTVCC captions. Thus, programs are often created and transmitted with both CC and DTVCC captions. Additionally, there is a multitude of content from the last several decades that was created with CC captions, but no DTVCC captions.
Captions and subtitles for video programs can be stored in other formats. For example, SAMI (Synchronized Accessible Media Interchange) is a caption format developed by Microsoft. SAMI is designed to support the creation of captions for media playback on a PC, and is better suited to interne streaming than cable or satellite broadcast purposes. SAMI captions are stored and transmitted separately from the video and are synchronized with the video using timestamps. SAMI captions are typically used when video is played back on a computer using a program such as Microsoft MediaPlayer. Similarly, subtitles are stored in various file formats and can also be carried in MPEG2 transport streams.
Teletext data can also be used for subtitles and captions. Teletext data can be carried in a number of ways, including within an MPEG2 transport stream, in various text and binary file formats, in lines of an analog PAL (Phase Alternating Line) video signal and/or in lines of a PAL SDI video signal. Typically, teletext data is encoded in multiple lines (5 or 10 per field) in an analog video signal. In a digital video stream, teletext data can be stored at a multiplexed transport stream level. For example, a multiplexed signal can include a video stream, an audio stream, and a teletext/subtitle stream, including teletext packets that are tagged with time stamps for synchronization. PAL does not provide a facility to carry CC data on line 21 of the signal, so the caption data must be either be translated into teletext data (which can be inserted into a PAL format electrical video signal) or subtitles which can be displayed as text on the video pictures.
Recently, the VOD (Video On Demand) industry has emerged. A VOD system provides a library of video programs from which the customer can choose. The customer selects a program, and it is delivered to the customer's VOD player over digital satellite, digital cable, and/or computer networks. To create the VOD library, the original programs have to be converted to a digital format that is compatible with the VOD player. This typically involves converting from analog to digital, encoding (also referred to as compressing), re-encoding or trans-coding the original program. Also in the last several years, two newer digital video formats have emerged along side MPEG2, and will likely replace MPEG2. These two formats are H.264 and SMPTE421M (VC-1). VOD content is being created in both of these new formats.